If the depth of the cab is worth stretching another inch vs a negligable difference. I do have that room to spare, so go ahead and use that to your advantage, if needed.

If you haven't started yet, don't worry about it. I miscalculated something and 48" is too tall :/ and I'm doubting the other small changes are enough to make it matter... Apologies in advance if you've already been plugging away.

Yeah, I agree. Might as well experiment with different sq ft openings to get a few extra LF dB like his image shows (posted a few pages back). Might not hurt to experiment with the TH18s I have as well.

I have some stoutly built bass bins of outside dimensions 40" H X 18"W x 24"D and wonder about building a 15" version. 5.5 inches too short for an 18 but a 15" would gain 3 inches of S2. Can I get away with an inside width of 16.5" instead of the 24" width of the 18" version since I'm running the same depth (actually, it is deeper by 1.5" so I can lengthen S2 by another 1.5 inches and get really close)

Yes, I'll struggle my way through horn response to get an idea, but wondered if the Keystone is sensitive to that kind of manipulation?
My initial though is to use the JBL GT5 15" car woofer which uses the
same motor as the GTO 1214--with a larger cone. Since it has a much smaller Vas than traditional PA speakers, can I assume it won't need to be as wide? Final amplification is just 200 watts so not much of a need for a BL product higher than it's rated 16.7 (I'm assuming)

Just a set of bins that do 32 to 150Hz is all I need for the garage, I do have the options of not using the JBL 15 but with 200 watts to play with, no need for B&C monsters either. I would like to generate
121 dB with two of them driven with two 200 watt amplifiers for sound testing at the local church so the closer, the better.

In summation, if I use a 15" and make the path the same length, the depth is the same--can I narrow the cabinet because of the smaller woofer or is that a no go for Keystones? I prefer the Keystone over the typical 18" BR since they are protected from various things behind the keystone panel and prevent teenagers from becoming fascinated by watching woofers stroke (not sure why they do but....) Part of my idiot proofing concept...

I have some stoutly built bass bins of outside dimensions 40" H X 18"W x 24"D and wonder about building a 15" version. 5.5 inches too short for an 18 but a 15" would gain 3 inches of S2. Can I get away with an inside width of 16.5" instead of the 24" width of the 18" version since I'm running the same depth (actually, it is deeper by 1.5" so I can lengthen S2 by another 1.5 inches and get really close)

Yes, I'll struggle my way through horn response to get an idea, but wondered if the Keystone is sensitive to that kind of manipulation?

In summation, if I use a 15" and make the path the same length, the depth is the same--can I narrow the cabinet because of the smaller woofer or is that a no go for Keystones?
Thank you!

The path length will be a bit shorter which will raise the F3 a bit.
That said, I had to reduce the exit size for the 15" to keep a similar LF response as the 2x12" and 18". Reducing the exit size further makes the cabinet go lower, but also drops the level.

Reducing cabinet width may actually be an improvement for a 15", but you will need to experiment with the exit size. As a starting guess, I'd reduce the exit dimensions by the same % as the cabinet width and height are reduced.

The path length will be a bit shorter which will raise the F3 a bit.
That said, I had to reduce the exit size for the 15" to keep a similar LF response as the 2x12" and 18". Reducing the exit size further makes the cabinet go lower, but also drops the level.

Reducing cabinet width may actually be an improvement for a 15", but you will need to experiment with the exit size. As a starting guess, I'd reduce the exit dimensions by the same % as the cabinet width and height are reduced.

Since you have the existing cabinet shells, might as well go for it.

Thank you so much!

As a garage system, the keystone will cover the 15 from flying parts, oil, beer and kids. As you say, I have the shells of stout 3/4" ply and noted the ratio is close to the 18" version. If I can get a gain of efficiency of 3dB over the stock 90dB of the JBL--anything extra is gravy. A pair of those with 200 watts each should be more than enough for BBQs and noise complaints.

Now to practice with horn response, gather up some spare wood and get those 15's on order. Thank you for your assistance and guidance and I'll start making sawdust in a week or two.